(1) FIELD OF THE INVENTION
The present invention relates to a continuous mill plant for rolling steel plates which is designed to work at the minimum rolling power.
(2) DESCRIPTION OF THE PRIOR ART
Electric motors in general include direct-current motors and alternating-current motors. To date, most of the motors which have been used in rolling mills have been direct-current motors, since sufficient frequency conversion techniques have not been developed for controlling the speed of alternating-current motors. However, increases in capacity of direct-current motors have been limited in terms of commutating ability.
The characteristics of such a conventional direct-current motor for rolling will be described with reference to accompanying drawings. FIG. 4 relates to one of the standard types of conventional continuous mill plant for rolling steel plates, namely, a 5-stand tandem rolling mill plant for producing cold-rolling steel plates of medium and increased thickness. The ordinate represents rolling speeds, and each number of the abscissa represents rolling mills. In this figure are depicted a line (lower limit) connecting the minimum rolling speed points and a line (upper limit) connecting the maximum rolling speed points defined at the continuous rated output of a motor for driving each rolling mill. The form of an area between the lower and upper limit lines in each figure is hereinafter referred to as a speed cone, and the ratio of maximum rolling speed to minimum rolling speed is referred to as the rolling speed ratio. The rolling speed ratio of a steel rolling mill plant is generally about 2.0 and less than 3.0, as shown in "Iron and Steel Manual" (Vol. 3) (2) (Nov. 20, 1980) edited by The Iron and Steel Institute of Japan, Maruzen, p. 1349. This value is due to the limitation in current rate of a direct-current motor based on the commutating ability described above.
From such speed cone characteristics of a rolling mill plant using a direct-current motor, a conventional method of, for example, producing cold-rolled steel plates involves a plurality of rolling mill plant rows such as rolling mill plants for processing thin and thick materials, respectively. The range of dimensions and qualities of a steel plate processed by each of these rolling mill plants are set to be comparatively narrow so as to correspond to a rolling speed ratio of less than 3.0. This arrangement has been necessitated by the need to produce different types of products of differing thicknesses.
The relationship between speed cone characteristic and degree of rolling is described below with respect to rolling mill plants for respectively processing thin and thick materials. In a rolling mill plant for thick material, a speed cone is such as shown in FIG. 6, since the ratio of the original plate thickness (of a material to be processed before rolling) to the product thickness after rolling, namely, the rolling reduction ratio, is small, as shown, for example, at Nos. 3 to 14 in Table 2, the difference between rolling speeds at the initial and final rolling mills thereby is small. Conversely, in a rolling mill plant for thin material having a speed cone such as shown in FIG. 7, the rolling reduction ratio is large, as shown, for example, at Nos. 1 and 2 in Table 2. In both cases, it is possible for material adapted to each design to be rolled within the area of speed cones, and the power of rolling mills is used efficiently.
On the other hand, when thick and thin materials are processed by a conventional rolling mill plant of either the type for processing thick material or the type for thin material with a view to eliminating or reducing the investment in labor and installations from the level currently needed, there is a problem of difficulty in performing rolling or of inefficient use of rolling mill power.
In a rolling mill plant for processing thick material and having such speed cone as shown in FIG. 6, when a thin material such as, for example, shown at Nos. 1 and 2 in Table 2 is rolled, the rolling speed is restricted to the upper limit of the speed cone at the final stand and so at each of the first to fourth stands even though there is some power margin. The rolling speed at the first stand is thereby reduced below the lower limit of the speed cone. Thus, the power of the rolling mill is not efficiently used, and the efficiency of production is considerably reduced compared with the rolling performed by rolling mill plants for processing thin material.
Conversely, when a thick material such as, for example, shown at No. 14 in Table 2 is rolled by a rolling mill plant for processing thin material and having a speed cone such as shown in FIG. 7, the rolling speed at all rolling stands can not be raised to the lower limit of the speed cone, the rolling itself thereby being extremely difficult.
Thus, in the conventional rolling mill plants, there are severe restrictions on the ranges of dimensions and qualities of a material to be rolled. There has not been any known practical techniques which would enable a single rolling mill plant to operate over the whole processing range without this defect.